The Rivers and Streams of England and Wales: An Overview of their Physical Character in 2007–2008 and Changes Since 1995–1996 - River Conservation and Management - page 27

with the 2007-2008 survey. The second baseline
survey (2007-2008) selected two sites from rivers
represented on the 1:250 000 scale river maps and
one from streams featured only on 1:50 000 scale
Ordnance Survey maps in each 10 km square.
This approach ensured that: (i) the sample was
representative not only of larger streams and rivers,
but also minor watercourses, some of which were
headwaters; and (ii) two-thirds of the sites were
selected using the same protocol as in 1995-1996,
ensuring that a direct comparison could be made.
The survey comprised 3166 sites at the 1:250 000
scale (the 'core' sample) and 1718 sites at the
1:50 000 scale (the 'minor watercourse' sample)
(Figure 3.1). The original baseline sites were
intentionally not re-surveyed because there was
little confidence that the site boundaries would
coincide exactly.
Comparisons between the two baseline surveys
used the 1995-1996 data and the core 2007-2008
sample. In a few instances, direct comparison is

not possible, either due to minor changes in survey
design (made in 1997 and 2003) or improved
training of surveyors since 1996. The main example
of improved training relates to greater emphasis
on the recognition of subtle diagnostic indicators
of bank and river-bed channel resectioning. This
has improved the confidence in recording this
particular type of modification, but means that
change in the extent of resectioned channel (a
major scoring component of HMS) has not been
included in the analyses.

Calculating the results

Statistics calculated from the RHS baseline surveys
need to take the stratification scheme into account
to produce unbiased estimates for England and
Wales. This applies both to the extent of features
(i.e. the proportion of river length with a habitat
or modification characteristic) and the prevalence

Figure 3.1 The 2007-2008 sampling strategy: (a) 'core' sites (solid circles) on the 1 250 000 scale river network-for
comparison with the 1995-1996 baseline; (b) 'core' sites, plus 'minor watercourse' (triangles) sites on the 1:50 000 scale
river network. Four 10 km squares are shown. © Crown Copyright and database right 2011. All rights reserved.
Ordnance Survey licence number 100026380. Some features of this map are based on digital spatial data licensed from
the Centre for Ecology and Hydrology, © CEH.